Improvements to "Bounty Goal met" - Expire goal if developer doesn't start work, show date goal was met

After a Bounty Goal has been met, the developer should be encouraged to start work on the issue.

We currently only email the developer that the goal has been met and ask them to click Start Work.

We should:
- show the date / time elapsed since the bounty goal was met
- email a reminder of "goal met" after 3 days, and a final notice at 7 days
- if the developer doesn't "Start Work" within x days (14?), we should remove the goal

Suggestion came a suppport@ ticket, referencing a Goal being met but no developer activity: https://www.bountysource.com/issues/1327922-a-fit-function-that-shows-only-the-text-on-a-page
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